July-August GSA Bulletin Media Highlights

The July/August GSA BULLETIN features studies on the eruptive processes recorded in Colorado’s San Juan Mountains; some of West Antarctica’s oldest known ice and the climatic record it might reveal; the effect of humidity on erosion, faulting, and asymmetry in the mountains of Columbia; the influence of clay on debris-flow hazards; northern Australia’s branching and growing Magela Creek; and the story of future earthquake potential as told by submerged paleoshorelines off the California Continental Borderland.

Highlights are provided below. Representatives of the media may obtain complimentary copies of articles by contacting Christa Stratton at . Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GSA BULLETIN in articles published. Contact Christa Stratton for additional information or other assistance.

Non-media requests for articles may be directed to GSA Sales and Service, .

***************Eruptive and noneruptive calderas, northeastern San Juan Mountains, Colorado: Where did the ignimbrites come from?

Lipman et al. present newly identified and reinterpreted ignimbrite calderas in the northeastern San Juan Mountains, southwestern Colorado, and document unique eruptive features not described previously from large volcanic systems. No voluminous tuffs vented directly from the Cochetopa Park caldera (20-km diameter); instead, this large caldera subsided passively as the more than 500-cubic-kilometer Nelson Mountain Tuff vented 26.9 million years ago from an "underfit" collapse area within the San Luis caldera complex 30 km to the SW. Tightly grouped isotopic ages document an exceptionally brief duration (50-100,000 years or less) for sequential eruption of three separate regional ignimbrites (each hundreds of cubic kilometers in volume) and interleaved caldera-filling lava flows from the San Luis complex; this recurrence rate for large caldera-related explosive eruptions is more rapid than previously documented elsewhere. In eruptive processes, volcanic compositions, areal extent, duration of activity, and magmatic production rates and volumes, Oligocene volcanic rocks (38-25 million years old) of the Southern Rocky Mountain region represent present-day erosional remnants of a composite volcanic field, comparable to younger ignimbrite terranes of the Central Andes.

***************Physical setting and tephrochronology of the Summit Caldera Ice Record at Mount Moulton, West Antarctica

A site on the shoulder of the ice-filled summit crater of Mount Moulton, located at an elevation of 2800 m in West Antarctica, exhibits a 400-m-long section of exposed blue ice and intercalated tephra layers. A total of 48 tephra layers are observed. Many are thick (up to 10 cm) and coarse (pumice up to 3 cm). Detailed dip measurements and global positioning system (GPS) mapping reveals a simple geometry of parallel tephra layers all exhibiting consistent dips. The simple stratigraphy of the tephra layers suggests that although the local ice is thinned, based on the boudinaged appearance of the thicker tephra layers, it is otherwise undeformed. Local measured ice motion and ablation rates are similar, averaging around 3 cm/yr for the 4-yr measurement period. Most of the Mount Moulton tephra layers are trachytic, and are derived from Mount Berlin, a still thermally active volcano about 30 km to the west. Using 40Ar/39Ar geochronology of potassic feldspar phenocrysts, Dunbar et al. directly dated eight of the tephra layers at the Moulton site. The depth-age curve obtained by Dunbar et al. for the Mount Moulton site, based on their radioisotopic ages, is qualitatively similar to that of the Siple Dome ice core, also in West Antarctica. Although not part of the West Antarctic ice sheet, the Mount Moulton site certainly contains some of the oldest known ice in West Antarctica and provides a long and detailed climate record.

Kruckenberg et al. present new U-Pb and Ar-Ar isotopic analyses that show that the crystallization of once partially molten rocks (migmatites), detachment tectonics (responsible in part for exhumation of deep crustal rocks), and basin evolution were coeval. The Okanogan gneiss dome is part of a belt of metamorphic core complexes that contain gneiss domes and that extends from British Columbia (Canada) into Washington state and Idaho (USA). Their new data document that Paleocene-Eocene partial melting affected a more widespread region of the northern Cordillera than previously recognized and suggest that this collapsed orogen may be analogous to active orogens that contain large volumes of melt (e.g. Tibetan-Himalayan orogen as a modern analogue).

***************Climatic forcing of asymmetric orogenic evolution in the Eastern Cordillera of Colombia

Mora et al. describe convincing new evidence about the interaction between highest erosion rates due to more humid climatic conditions, faster movement of faults, and consequent asymmetry in mountain belts. These interactions are documented with accurate structural data, apatite fission track ages, and paleoelevation histories. The study area is the Colombian Eastern Cordillera, an orogen located in a tropical region where precipitation is concentrated on one side of the orogenic belt. Mora et al. propose that such higher precipitation was in place during the Plio-Pleistocene and favored faster rates of movement along the main boundary faults of the more humid side of the range and therefore higher shortening and structural relief.

Crete and Karpathos (Greece) are located on the southwestern part of the Aegean-Anatolian plate. Reconstruction of vertical motions over the past 7 million years reveal that these islands started to be uplifted two millions of years earlier than generally believed, at ca. 5 Ma, by which most of the approximately 1 km tectonic uplift occurred between 5 and 3 Ma. Uplift of these islands has always been puzzling because the Aegean lithosphere has been stretched several hundreds of km over the past 20 m.y. due to the southward retreat of the subducted African lithosphere, which, as a result, should lead to continued subsidence instead of uplift of Crete and Karpathos. Uplift has been explained by collision between the southern margin of the Aegean lithosphere and continental Africa but is untenable in view of GPS data indicating that Crete still moves southward as well as the absence of compressional structures. Zacariasse et al. relate uplift to a recently published scenario in which a tear in the subducted African slab migrates laterally, causing isostatic uplift of the overriding Aegean lithosphere at the leading edge of this tear. Their reconstructed amount of uplift and the timing of maximum uplift are in good agreement with model results. Increased seismicity and tilting of fault blocks in the southern Aegean region associated with isostatic rebound of and horizontal displacements in the southern Aegean lithosphere also provide an explanation for the puzzling mass-wasting deposits found all over Crete and Karpathos and offshore, which, as they discovered, were emplaced over a period of 1.35 million years, corresponding with early uplift.

Efficient extraction of granitic magma from crustal sources requires the development of an extensive permeable network of melt-bearing channels during deformation. Weinberg and Mark investigate rocks that have undergone deformation and melting within the Karakoram Shear Zone of Ladakh, northwest India, in which leucosome distribution is inferred to record a permeable network for magma extraction. Delicate structures preserved in these rocks record the development of this permeable magma network and its subsequent destruction to form a mobile mass of melt and solids, resulting from the interplay between folding and magma migration. During folding, magma migrated from rock pores into layer-parallel and axial-planar sheets, forming a stromatic migmatite or metatexite with two communicating sets of sheets, intersecting parallel to fold axis. Once the network was developed, folding and stretching was eased by magma migration and slip along axial planar magma sheets. Folding and magma migration led to layer disaggregation, transposition, and the formation of a diatexite where rock coherency and banding was destroyed. A number of structures developed during this process, such as cuspate fold hinges, disharmonic folds, truncated layering, shear along axial planar leucosomes, and flow drag and disruption of melanosomes. In this system, magma migration was an integral part of deformation and assisted the folding and stretching of metatexites, while folding gave rise to a magma sheet network, now preserved as leucosomes, as well as the pressure gradients that drove magma migration and the breakup of the metatexite. Thus, metatexite folding increased melt interconnectivity, while magma mobility increased strain rate and released differential stresses.

***************Holocene debris flows on the Colorado Plateau: The influence of clay mineralogy and chemistry

Debris flows occur in areas of the Colorado Plateau where certain types of bedrock outcrop on steep slopes. Webb et al. found that bedrock associated with debris flows is high in single-layer clays, such as kaolinite and illite, and bedrock that does not regularly produce debris flows is high in multilayer, swelling clays. Their work may be a basis for debris-flow hazard delineation in arid and semiarid regions with high topography and bedrock either containing or weathering to single-layer clays.

***************Riparian vegetation and the late Holocene development of an anabranching river: Magela Creek, northern Australia

Anabranching rivers consist of multiple channels that divide and rejoin around stable islands. Island formation commonly occurs in close association with riverine vegetation growth (trees, shrubs, grasses), but many uncertainties surround the processes and timescales involved. Tooth et al.'s study used geomorphological investigations and optically stimulated luminescence (OSL) dating to investigate anabranching development along a 6.5-km-long reach of Magela Creek in the seasonal tropics of northern Australia. Over the last few thousand years, the creek has extended in length by building a sandy delta into a shallow lake (Madjinbardi Billabong), so that a time sequence of islands and anabranches exists along the study reach. Islands in the upstream and middle parts of the reach are at least 1,600 years old, and here anabranches are relatively efficient conduits for water and sediment transport, being narrow and deep with little obstructing within-channel vegetation. Farther downstream, islands are younger, and anabranches are less efficient, being wider and shallower with more obstructing vegetation, and localized erosion is common. In these downstream parts, however, the most inefficient anabranches tend not to persist, as they either infill with sediment, or are subdivided into more efficient anabranches by new islands that form in association with colonizing vegetation. As more efficient anabranches develop, downstream sediment transport increases, which enables ongoing delta growth, and provides fresh sediment surfaces upon which more islands eventually form. Tooth et al.'s findings demonstrate that a relatively efficient anabranching system can develop within a few thousand years, and they contribute to the growing body of research that highlights the key influence of vegetation on many aspects of river process, form, and behavior.

Constraints on the polarity of Cretaceous subduction in the Greater Antilles are provided through geochemical comparison between the erupted island arc lavas in central Puerto Rico and potential pelagic sediment reservoirs in the flanking ocean basins. Compositions of Puerto Rican arc basalts are inconsistent with incorporation of Pacific pelagic chert. Instead, patterns characteristic of high-Fe island arc tholeiites are reproduced by incorporation of up to 4% of a low-Zr/Sm biogenic sediment component of Atlantic origin, whereas patterns of low-Fe lavas require, in addition to biogenic sediment, introduction of up to 2% of a high-Zr/Sm crustal turbidite component. The Atlantic origin of all the subducted sediments indicates the polarity of subduction throughout the Cretaceous in the northeast Antilles was persistently southwest-dipping. Jolly et al.'s conclusion is supported by the presence of a low-Zr/Sm supra-subduction zone component of Atlantic origin in Caribbean plateau basalts (91-88 Ma) from southwest Puerto Rico, which were erupted within the broad backarc region of the Greater Antilles during intermediate stages of arc development.

In southern California and other highly populated regions, large earthquakes at the intersection of active structural domains pose a significant hazard. An understanding of the deformation history and the future earthquake potential of the individual faults within the intersecting structural systems is critical in determining the seismic hazards that these regions pose. Chaytor et al. report on the use of submerged paleoshorelines as strain markers to investigate Holocene and late Pleistocene vertical tectonic movement at the intersection of the offshore Santa Cruz-Catalina Ridge with the southern boundary of the western Transverse Ranges, within the California Continental Borderland. An uplift rate on the order of 1.50 +/- 0.59 mm/yr over the past 2300 years of the easternmost of the northern Channel Islands determined from intertidal and subtidal shells, closely matches published slip-rates for the underlying Channel Islands thrust. Although there appears to be a significant component of underthrusting occurring at the intersection of the Western Transverse Ranges and Borderland provinces along the Channel Islands thrust interface, it may only represents a small fraction of the total required to accommodate northward motion of the Borderland block.